In the production of aluminum bodies it is a common practice to utilize, as much as possible, aluminum scrap to reduce the quantity of raw materials required. Such scrap normally is associated with a relatively high proportion of various impurities which must be removed before the scrap aluminum forms a useful melt. The scrap aluminum is treated in aluminum remelting plants, generally in a rotary furnace, to remove these impurities.
To assist in the removal of the impurities, the common practice is to add slag-forming salts to the scrap aluminum in the furnace.
One part of the salt mixture is added to two parts of the aluminum scrap in the smelting furnace. The salt mixture generally comprises 25 to 30% by weight potassium chloride, 65 to 70% by weight sodium chloride, 2% by weight calcium fluoride (CaF.sub.2) and traces of other chlorides, fluorides, sulfates and bromides.
Aside from acting as slag formers, the additive also controls the rheology of the melt.
Because of the remelting of aluminum scrap, large quantities of salt slags are obtained which can be constituted of 4% to 8% by weight aluminum metal, 18% to 20% by weight potassium chloride, 45% to 50% by weight sodium chloride and 22% and 33% by weight of water-soluble components.
The disposal of these slags with other wastes creates serious ecological problems since it can result in an increase in the salt concentration of ground water and, upon solubilization of the salts, an evolution of gases which are partially toxic and are noxious. The storage of these wastes in other ways, i.e. in subterranean caverns, has been found to be highly uneconomical.
Experiments have been carried out to treat aluminum salt slags to recover the aluminum and the salts.
For example, a solubilization process has been developed which involves high treatment cost and large energy expenditures. Other salt recovery processes, such as reverse osmosis, solubilization and freezing or chemical or thermal precipitation, have not proved to be practicable also because of their high energy costs and their environmental effects.
Efforts to carry out a separation by high voltage electrostatic precipitation techniques do not yield the desired results.